The vertebrate spinal column includes a series of vertebrae extending from the skull to the small of the back enclosing the spinal cord and providing support for the thorax and abdomen. Each vertebra is an irregular bone with a complex structure composed of bone and some hyaline cartilage. The proportions of each vertebra can vary according to the location in the spinal column and between individual animals.
The basic structure of a vertebra comprises a vertebra body including a main anterior bony part that consists of the centrum, the ossified posterolateral joints linking the centrum and each half of the neural arch, and part of the neural arch. The upper and lower surfaces of the vertebra body give attachment to the intervertebral discs. The posterior part of a vertebra forms a vertebral arch, consisting of two pedicles, two laminae, and seven processes. The laminae give attachment to the ligamenta flava. There are vertebral notches formed from the shape of the pedicles, which form the intervertebral foramina when the vertebrae articulate. These foramina are the entry and exit conduits for the spinal nerves. The body of the vertebra and the vertebral arch form the vertebral foramen, the larger, central opening that accommodates the spinal canal, which encloses and protects the spinal cord. The spinous process is directed backward and downward from the junction of the laminae, and serves for the attachment of muscles and ligaments. The articular processes, two superior and two inferior, spring from the junctions of the pedicles and laminae. The superior project upward, and their articular surfaces are directed more or less backward; the inferior project downward, and their surfaces look more or less forward. The transverse processes, two in number, project one at either side from the point where the lamina joins the pedicle, between the superior and inferior articular processes. They serve for the attachment of muscles and ligaments. Vertebrae articulate with each other to give strength and flexibility to the spinal column, and the shape at their back and front aspects determines the range of movement.
Spinal manipulative therapy is a “hands-on” treatment of the spine in which the therapist moves the vertebrae within the physiological range of motion. The therapist may use slow, passive movements, starting with a small range and gradually increasing to a larger range of motion or the therapist may apply specifically directed manual impulse, or thrust, to a joint, at or near the end of the physiological range of motion. Typically, spinal manipulative therapy includes posteroanterior mobilization of the vertebra largely confined to the sagittal plane of the spine and rotational mobilization which involves complex three dimensional movements and loading resulting in axial rotation and movement in other planes. The spinal posture during spinal manipulative therapy can have a significant influence on the movement of the spine. Lee, Raymond Y. W., Kinematics of rotational mobilization of the lumbar spine, Clinical Biomechanics 16 (2001) 481-488.
Accordingly, a wide variety of devices are available to assist in fixing the spinal posture during spinal manipulative therapy. As an illustrative example, mobilization wedges may utilized to orient the articular surfaces for passive or active joint mobilization. Typically, a mobilization wedge will have a thick end and a thin end joining two principal flat faces meeting in an acute angle. For spinal mobilization, the wedges often have a cut out disposed axially and medially in one of the two faces. The cut out provides a space in which the spinous process of a vertebral segment can be located while the edges of the cut out which join the flat face block the transverse processes.
The structure of conventional mobilization wedges may introduce shortfalls and risks in spinal manipulative therapy. A first aspect can be that the height of the thick end joining the two principal faces of the mobilization wedge may allow excessive posteroanterior mobilization of the vertebra in the sagittal plane between supported and unsupported spinal segments. A second aspect can be that that the flat face of the wedge extending outward on opposite sides of the cut out terminates axially at the thick end of the wedge. Thus, in an adjoining pair of vertebrae disposed proximate the thick end of the wedge, a first vertebrae supported by the wedge can have both transverse processes supported by the flat face on either side of the cut out while the second vertebrae unsupported by the wedge can have neither transverse process supported by the face on either side of the wedge. Accordingly, the rotational movement of the unsupported vertebrae can be in either direction about the axis of the spinal column when the therapy is directed only in one direction about the axis of the spinal column. A third aspect can be that the wedge may not include any supporting member which extends outward axially or laterally from the thick end of the wedge.